Heat exchangers are utilized in a variety of industrial processes to transfer heat between two or more fluids by indirect heat exchange. There are different designs for heat exchangers. For instance, one heat exchanger design is commonly referred to as a shell and tube design in which one fluid flows through the tubes and another fluid flows outside the tubes but inside a shell housing that retains the tubes. The fluid can be a liquid, vapor or a combination thereof. Further, the shell can be formed by or integrated with other equipment in which the heat transfer is to be conducted, such as, for example, a distillation column.
In another type of design, known as a plate-fin heat exchanger, a series of plates, referred to commonly as parting sheets, are connected at their respective edges by end bars and fins to enhance the heat transfer between the plates. Header tanks connected to the plates introduce the process or working fluids into the passages formed between the plates to accomplish the indirect heat exchange between the fluids.
Where one of the fluids is a liquid to be boiled at a boiling side surface of the heat exchanger, a porous coating can be used along the boiling side surface to promote the increase of heat transfer through a given surface, per unit surface area, (i.e., heat flux) in response to a given temperature difference between the heat source input and the fluid to enable the fluid to boil. The term “porous coating” as used herein and throughout refers to those coatings that by virtue of its built-in porosity enhance boiling by providing so-called nucleation sites. The porous coating provides micro-scale cavities that have the effect of increasing the number of nucleation sites and bubble departure frequency per site. As a result, the boiling rate can be enhanced.
However, currently available porous coatings, such as those mentioned in U.S. Pat. No. 4,917,960, are inefficient, particularly for cryogenic boiling heat transfer applications. Heat transfer efficiency is generally used to assess the performance of the porous coatings. As used herein and throughout the specification, the performance is defined by a temperature difference, ΔT, that is equal to T1−T2, where T1 is defined as the temperature of the heat source input and T2 is defined as the temperature of the process fluid to be heated to its predetermined temperature (e.g., boiling point). A coating with a relatively lower ΔT would be considered better performing, by virtue of its ability to promote greater heat transfer to the process fluid for a given heat source input (e.g., a gas located on the shell side of a shell and tube heat exchanger design having a temperature, T1, greater than that of the process fluid, T2, flowing within the tube of the shell and tube heat exchanger). Improved performance is defined at least in part by a reduction in the ΔT. It should be understood that heat transfer efficiency as used herein and throughout the specification may be used to assess coating performance for boiling heat transfer applications. As will be explained in greater detail below, heat transfer efficiency will be used to assess coating performance of porous coatings for various applications, including boiling heat transfer applications, whereby heat is transferred from a heat source or heat source input to a fluid to cause boiling of the fluid. For boiling heat transfer applications, the term “boiling ΔT” may be used herein and throughout. It should be understood that the term “ΔT” and “boiling ΔT” may be used interchangeably herein and are intended to have the same meaning.
Generally speaking, conventional porous coatings suffer from an unacceptably high ΔT. In other words, a large amount of heat energy is required to be transferred to the boiling surface to boil the process fluid, which translates into inefficient processes having excessive power consumption.
These conventional porous coatings do not sufficiently increase or enhance heat transfer efficiency to the boiling surface, as required by today's more demanding applications. Further, such conventional porous coatings may not have the ability to facilitate the onset of nucleation boiling, by, for example, increasing the number of nucleation sites for boiling to occur. Generally speaking, the heat transfer and especially the boiling heat transfer increases in direct proportion to the number of active bubble column sites.
Today's heat transfer coatings have attained a maturity level where further increase in performance, efficiency and operational cost savings are not technically feasible. In view of these shortcomings, and the ever increasing need for improved performance, there is a need for new generation coating compositions that can enhance heat transfer for various applications.